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[mkgmap-dev] r4006: 1st alpha version to write DEM data

From Gerd Petermann gpetermann_muenchen at hotmail.com on Wed Dec 20 09:12:09 GMT 2017

Hi Andrzej,

I looked at the three demo topo maps that I have which have more that one zoom level and I found this:
All have the same levels 24,22,20,18.
topo2 uses DEM point distances 1648, 6624, 26512, 106048  and was created  17 Dez 2007
Transalpin uses DEM point distances 3312, 13248, 26512, 53024  and was created  2 Sep 2009
topo-2010 uses DEM point distances 5520, 16560, 44178, 88368 and was created  28 Apr 2010

The distance between two DEM points is 360° / 2^32  or ~40000km / 2^32 ~ 0,00931 m (lat), so 3312 *0,00931 m is ~ 30 m.
So, I see no obvious rule for the ratios, esp. there are not always exact multiples, so we probably need an option for this.
I see no need to store the ratio in an extra field in the header.


Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Andrzej Popowski <popej at poczta.onet.pl>
Gesendet: Mittwoch, 20. Dezember 2017 00:28:59
An: mkgmap-dev at lists.mkgmap.org.uk
Betreff: Re: [mkgmap-dev] r4006: 1st alpha version to write DEM data

Hi Gerd,

great news!

 > It will crash if any hgt file is missing
I have them all, but there is no HGT for sea area, missing files is a
standard case.

 > I did not check if the flags in the tdb file are set correctly now
I didn't check either, but tdb created by mkgmap v3999 works correctly
for maps with DEM.

 > My currennt understanding is that mkgmap should do the calculation of
 > the DEM resolution based on the levels used to create the rest of the
 > map.
I think it should be like that and some Garmin maps follow this rule. I
have tried to create DEM with pixel size ratio like 1:4:16:64 for levels
24,22,20,18. It didn't work correctly in Mapsource. What works for me is
ratio 1:2:4:8. I think some of unknown fields in DEM header could
contain this ratio. It is something to investigate further.

Best regards,

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